The present invention relates to gear driven speed reducers, and more particularly, to parallel shaft, double stage speed reducers primarily used with electric drive motors in mixer assemblies.
Speed reducers, known simply as "reducers," are attached to the output shaft of an electric drive motor to provide a low speed, high torque output shaft. Such reducers typically are interposed between the electric drive motor and the shaft of an impeller in a low speed mixer. One type of reducer comprises a housing having a plurality of parallel, vertically-oriented shafts, each carrying a gear. An input shaft is connected to the output shaft of the electric drive motor, and its gear meshes with a larger diameter gear on an intermediate shaft. A smaller gear on that intermediate shaft meshes with a large diameter ring gear connected to an output shaft. This low speed ring gear is mounted on a flange so that it extends below the lubricant level in the housing.
Typically, a cylindrical dry well is formed in the housing and surrounds the output shaft. This dry well forms a lubrication sump with the housing. Bearings and gears not able to be positioned within the lubrication sump are lubricated by "splash" lubrication or by use of a pump mechanism, which pumps lubricant to such exposed bearings and gears.
With such lubrication systems, steps must be taken to prevent the lubricant from leaving the lubrication sump and possibly fouling the materials being mixed by the associated impeller. Likewise, steps must be taken to prevent the materials being mixed, or other environmental liquids or solids, from entering into the gearbox and thereby contaminating the lubrication fluid. For these reasons, many conventional reduction drive assemblies incorporate complicated seal and dry-well systems with the output shaft assembly.
Additionally, to withstand loading in the range of 10,000 in-lb of torque and 20,000 in-lb of bending moment, the output shaft of a drive assembly should be high strength steel, typically 2 to 3 inches in diameter. With such a large diameter output shaft, the diameter of the coupling should be on the order of 21/2 times the shaft diameter. A difficulty with providing such a large diameter output shaft and coupling is that the drive assembly must incorporate the dry-well about the shaft and lower, low-speed output shaft bearing, while still fitting the dry well under the gear attached to the output shaft. Another problem is that it is difficult to design such a reduction assembly where the low-speed bearing and/or the dry-well will not interfere with the supports and bearings for the output drive shaft and intermediate pinion. If the pinion on the intermediate shaft needs to be raised to accommodate the large dry-well and associated low-speed bearings, there is a difficulty in lubricating the upper bearing on the intermediate shaft with the sump oil, without incorporating complicated lubrication pumping mechanisms. Accordingly, most vertical parallel shaft reducers with dry-well seals lubricate the gear mesh by means of an oil pump. A disadvantage with such a design is that the oil pump not only adds a degree of complexity to the drive but also reduces reliability.
Furthermore, to accommodate dry-wells large enough to surround the large output shaft and bearing assembly, many conventional drive assemblies separately press the dry-well into place and glue the components closed. Therefore, such conventional drive assemblies must rely on tolerances on the separate dry-well to be sufficiently close and that the adhesive will not fail.
Another problem with conventional reduction drive assemblies is that many conventional drive assemblies place the upper, and/or lower low-speed bearings against portions of the gearbox, such as the dry-well, that do not have substantial strength. Therefore, it has been known in mixing applications with high bending loads that these portions of the gearbox bearing such bending loads from the output shaft and associated low-speed bearings have been known to fail.